1. Field of the Invention
The invention concerns microwave phase shifter circuits and, more particularly, microwave phase shifter circuits that achieve elementary phase shifts under the control of electrical signals and can be grouped together to obtain phase shifts that are whole number multiples of elementary phase shifts. In microwave applications, transmission lines that may take different forms are used. FIGS. 1 to 4 respectively show views that represent a slot transmission line and a strip transmission line, in this particular case a microstrip transmission line.
A slot line (FIGS. 1 and 2) is formed by an aperture 1 made in a metallic layer 2, deposited on a dielectrical substrate 3. The dielectrical substrate ensures the mechanical strength of the metallic conductors and forms the transmission medium of the microwave, the energy of which is concentrated between the edges 4 and 5 of the slot. In FIG. 2, the lines of force of the electrical field E have been shown in dashes, and those of the magnetic field in solid lines. The thickness of the dielectric material is related to its nature, and the width of the slot line determines the characteristic impedance of the line.
The strip transmission line (FIGS. 3 and 4) has a dielectric plate 7 placed between a strip 6 and a metallic plane 8, also called a ground plane. As for the slot line shown in FIGS. 1 and 2, almost all the energy is concentrated in the dielectric 7. In FIG. 4, the lines of force of the electrical field E are shown in dashes. The dielectric materials used in the two slot or strip lines may be polytetrafluorethylene, a beryllium oxide, an aluminium ceramic, a quartz or a ferrite.
In addition to having a function of transmission, the slot or strip lines may separately perform a phase-shifting function, in being configured in different ways. Thus, in implementing a strip line type of technology, it is possible to make so-called switched-line phase shifters, disturbance phase shifters or 3 dB/90.degree. coupler phase shifters.
For a description of these prior art phase shifters, whether of the slot line type or the strip line type, reference could be made to several articles in the Journal IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES and, especially, to an article by Elio A. MARIANI et al., "Slot Line Characteristics", Vol. MTT-17, No. 12, December 1969, pp. 1091 to 1096, as well as an article by J. F. WHITE, "High Power p-i-n Diode Controlled Microwave Transmission Phase Shifters", March 1965, pp. 232 to 242.
In certain applications, it is necessary to change from one type of line to another and, to this effect, numerous coupling devices have been proposed. These coupling devices are generally not designed to achieve, at the same time, a phase shifter circuit for which the phase shift value would be controlled. When a determined phase shift has to be introduced, the assembly is modified to insert a phase shifter circuit of any known type before or after the coupling device, thus increasing losses, complicating the assembly and making it bulkier. There are, therefore, applications where coupling devices need to have incorporated phase shifters or, conversely, applications where phase shifters need to be associated with coupling devices.